Little trivia, but Path of Exile 2 (videogame) has a map that's quite inspired by Aztecs and one recent reminder I had of their brutality was precisely running through this level called Aggorat and witnessing the mesmerizing gore surrounding everything.
Second of all, I want to talk about this:
> Perhaps the Mexica themselves are part of that tiny sliver, and out there in the modal logic æther are stable, flourishing civilizations more bizarre than anything we can imagine.
If "bizarre" involves this level of brutality, I'm not too persuaded. As TFA brings up, the Aztec empire lasted only ~200 years, and that short lifespan was in good part due to the brutality.
All the smaller societies around the Aztecs hated them, for obvious reasons, and also the Aztecs were actually pretty bad at war, as Clendinnen discusses. They were focused on incapacitating enemy warriors, not killing them, and had essentially no coordination in battle; the "army" was a mob of warriors each trying to take as many captives as possible. Taking captives was the whole point of war.
Obviously, this kind of behavior can only persist as long as there are a bunch of smaller societies to kick around. The Aztecs were actually quite weak militarily for their size and everyone around them was chomping at the bit to gang up and erase them from existence (which is what happened when the Spanish arrived). These are not the conditions for a stable, flourishing civilization, and they're a direct result of building society around brutality and killing for the sake of killing.